This invention relates to adjustable tuning apparatus preferably located at the input or antenna stages of a radio or similar signal processing apparatus. Prior art adjustable filters require that center frequency and bandwidth be predetermined by parameter settings, manually, mechanically, or electrically adjusted, or that input and output signals be sampled and processed to thereby determine tuning with respect to an applied reference signal. However, whenever it is desirable to automatically tune and track very weak applied signals, for example antenna signals at a radio receiver, it is not practical to sample the input signal.
Further, whenever regenerative processes are employed, such that weak signals are amplified and reinforced and reapplied to the tuning apparatus as in a regenerative receiver (Armstrong, DeForest), or in a high Q narrow bandwidth system (Dimon), stability and tuning with respect to an incoming signal is extremely difficult to achieve, even with a trained operator continually making tuning and circuit adjustments. This invention provides independent center frequency, bandwidth and signal level adjustments in accordance with signals being processed therethrough, including such signals as may be too weak to be ordinarily available for sampling. This invention applies to antenna tuning systems, receiver front end filters, Yttrium Iron Garnet (YIG) resonators, saturable core reactors, varicap diodes, tuned amplifiers, regenerative amplifiers and detectors, tuned bridge systems, such as the Wien, Meacham, resonant and twin tee bridges, wherein the center frequency is to be automatically controlled in accordance with an applied signal, the bandwidth is to be predetermined, and signal level controlled.